Why do DATEADD and DATE_ADD differ?

They are different vendor implementations of the same idea. SQL Server uses DATEADD(unit, n, date), MySQL uses DATE_ADD(date, INTERVAL n unit) and PostgreSQL uses plain interval arithmetic like date + INTERVAL '7 days'. The SQL standard never settled one spelling.

Does NOW() return UTC or local time?

It depends on the engine and session settings. PostgreSQL now() respects the session time zone, MySQL NOW() uses the server time zone, and SQLite datetime('now') returns UTC unless you add the 'localtime' modifier.

How do I truncate a date to the start of the month?

PostgreSQL has DATE_TRUNC('month', d) and SQL Server 2022+ has DATETRUNC(MONTH, d). MySQL has no equivalent, so you format to a fixed-day string like DATE_FORMAT(d, '%Y-%m-01'), and SQLite uses date(d, 'start of month').

Why does day-of-week return different numbers?

The numbering convention is not standard. MySQL DAYOFWEEK returns 1-7 with Sunday as 1, PostgreSQL and SQLite return 0-6 with Sunday as 0, and SQL Server depends on the SET DATEFIRST setting. Always normalise before comparing across engines.

Is anything sent to a server?

No. This is a static reference rendered entirely in your browser. There is no database connection and nothing you type is transmitted or stored.

SQL Date/Time Functions Reference

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One concept, four spellings

Date and time handling is where SQL portability breaks down fastest. Every major engine has its own function names, argument order and timezone defaults for the same logical operations. This reference puts the common tasks — current time, adding intervals, extracting fields, truncating and formatting — side by side across MySQL, PostgreSQL, SQLite and SQL Server so you can translate at a glance.

How it works

Each task maps to one expression per dialect. The differences fall into a few predictable buckets:

-- Add seven days
DATE_ADD(d, INTERVAL 7 DAY)   -- MySQL
d + INTERVAL '7 days'         -- PostgreSQL
date(d, '+7 days')            -- SQLite
DATEADD(DAY, 7, d)            -- SQL Server

Watch three things: argument order (SQL Server puts the unit first), the interval syntax (a keyword in MySQL, a string in Postgres), and whether the result is a date, datetime or text value. Functions like strftime in SQLite always return text, so wrap them in CAST(... AS INTEGER) when you need a number.

Conventions that bite

Day-of-week numbering is the classic trap: Sunday is 1 in MySQL, 0 in PostgreSQL and SQLite, and configurable in SQL Server via SET DATEFIRST. Unix-epoch extraction also overflows a 32-bit INT past the year 2038 in SQL Server’s DATEDIFF(SECOND, ...), so use DATEDIFF_BIG there. When in doubt, confirm timezone defaults — NOW() is local in some engines and UTC in others.